This application claims priority of European Application No. 00202821.5, filed on Aug. 11, 2000.
The invention relates to an aqueous-based cross-linkable binder composition and to its use in the production of lacquer coatings.
1. Field of the Invention
Aqueous cross-linkable binder compositions comprising aqueous polyurethane dispersions are well known for the production of film coatings for various substrates such as metal and plastics. Though high-quality coating layers can be obtained with the known aqueous-based binder compositions, the properties of the aqueous polyurethane dispersions used in the known compositions are still a subject for improvement, especially in the field of automotive refinishing shops. Such improvement may concern reducing their excessive foaming when applied via spraying, the amount of organic solvent (VOC), the toxicity of one or more of the components in the binder composition, and/or the curing speed at ambient temperature (RT).
2. Discussion of the Prior Art
It should be noted that aldehyde-terminated polyurethanes have been disclosed earlier in U.S. Pat. No. 3,392,148. The cured resins are said to be useful as protective coatings, mouldings, and adhesives for laminates and the like. The compositions are prepared by the addition reaction of hydroxy-aldehydes with polyisocyanates. The non-aqueous compositions are cured with the aid of an acid catalyst or with a material which will liberate an acid on heating up to about 100-200xc2x0 C. Not the slightest allusion is made to the fact that aqueous-based cross-linkable binder compositions curable at ambient temperature can be obtained by starting from an aldehyde-terminated polyurethane having a number average molecular weight of more than 1,000 and an average aldehyde functionality of xe2x89xa72 which comprises ionic and/or non-ionic dispersing groups, and adding a low-molecular weight cross-linker containing aldehyde reactive groups.
EP-A-0 332 326 discloses an aqueous self-cross-linkable coating composition comprising an aqueous dispersion which comprises at least one polyurethane polymer with hydrazine-functional groups and carbonyl-functional groups. The carbonyl-functional groups may be present in the same polyurethane polymer as the hydrazine-functional groups or not.
EP-A-0 367 051 discloses aqueous storage-stable, carbonyl groups-containing cross-linkable polyurethane dispersions. The carbonyl groups may be derived from hydroxyacetone, hydroxybenzaldehyde, acetoin, benzoin, adducts of diepoxides to ketocarboxylic acids. The polyurethanes are dispersed in water in the presence of ammonia or organic amines such as trialkyl amines, e.g. trimethyl amine, triethyl amine, or hydroxyalkyl amines such as trishydroxyethyl amine or mixtures thereof. For the cross-linker use is made of polyhydrazides. In using such dispersions for coating applications, however, coatings result with an appearance that is subject to improvement. Besides, polyurethanes based on xcex2-hydroxyketones can become unstable over time and decompose during storage with elimination of CO2.
EP-A-0 442 652 discloses aqueous coating compositions comprising aqueous polyurethane dispersions in which the polyurethane polymer has chain-pendant or in-chain groups Y with, in all examples, a chain-extending compound having three or more independently reactable enolic carbonyl groups, so that those not taking part in the chain extension will end up as lateral carbonyl-containing groups in the chain-extended polymer. The only chain-extender compound used in the examples is prepared from trimethylol propane triacetoacetate. Not the slightest allusion is made to the substitution of aldehyde groups for said triacetoacetate groups providing the composition with an improved water stability.
EP-A-0 584 818 discloses an aqueous self-drying and, optionally, cross-linkable coating composition for, e.g., multilayer lacquering of, e.g., repaired cars. The coating composition comprises an aqueous dispersion of a polyurethane resin of number average mol. wt. (Mn) between 2,500 and 1,000,000 containing at least one CH acid group per molecule. The cross-linker comprises an at least 2-functional aldehyde or a free or blocked polyisocyanate with on average 1.5 isocyanate functions per molecule. Preferably, the aldehyde is formaldehyde and/or glyoxal and the polyisocyanate is diisocyanate.
WO 96/41833 discloses a binder composition comprising a strongly activated carbanion-functional polymer and an aldehyde groups-containing cross-linker.
The invention now provides binder compositions which have an acceptable foaming behaviour and curing speed at ambient temperature, which compositions comprise
(A) an aqueous dispersion of an aldehyde-functional polyurethane having a number average molecular weight of more than 1,000 and an average aldehyde functionality of xe2x89xa72, which polyurethane comprises ionic and/or non-ionic dispersing groups, and
(B) a low-molecular weight aldehyde-reactive cross-linker selected from the group of low-molecular weight polyamines and low-molecular weight compounds comprising at least one group of one of the formulae E1xe2x80x94CHR1xe2x80x94E2 and Hxe2x80x94Cxe2x80x94 (E1E2E3), wherein xe2x80x94E1, xe2x80x94E2 and xe2x80x94E3 are independently chosen from electron-withdrawing groups such as xe2x80x94P(xe2x95x90O)xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CN, xe2x80x94SO2xe2x80x94, xe2x80x94NO2 and wherein R1 has the meaning of hydrogen or a hydrocarbon radical having 1 to 10 carbon atoms.
Preference is given to a cross-linker comprising acetoacetate groups.
The carbanion-functional groups preferably are acetoacetate groups, malonate groups, acetonate groups or mixtures thereof. The polymer is an alkyd resin or a polyester resin.